Physico-chemical limnology during the filling phase of Tugwi-Mukosi, a tropical reservoir in Zimbabwe

Abstract

The physico-chemical limnology and trophic status of the Tugwi-Mukosi reservoir were assessed seven months after impoundment. Water temperature, conductivity, pH, dissolved oxygen, transparency, nitrates, total nitrogen, orthophosphates, total phosphorus and metals (K, Na, Ca, Mg, Cu, Cd, Cr, Ni, Hg, Mn, Zn, Pb, Fe and As) were measured at five sites distributed across the Tugwi-Mukosi reservoir in July 2017. The reservoir was turbid after impoundment and had low water clarity. The trophic state of the reservoir ranged from eutrophic to marginally hypertrophic with high total phosphorus and total nitrogen concentrations. All measured physico-chemical water variables did not differ significantly between the lake sites with the exception of conductivity, ammonia and total phosphorus. Potassium, Na, Ca and Mg were high in the reservoir, whereas other heavy metals recorded low concentrations. Iron was relatively high with an average concentration of 0.24 mg l⁻¹. The average Ca:K, Ca:Na and Ca:Mg ratios in the reservoir were 3.8, 1.1 and 3.2, respectively. The high Ca:Mg of 3.2 might favour the establishment of snails. Eutrophication of the Tugwi-Mukosi Reservoir should be abated through coordinated management of the reservoir and the terrestrial environment in the catchment to ensure that the water quality is able to support lake biodiversity and confer ecosystem services to human livelihoods around the reservoir.

Keywords: aging, artificial reservoir, eutrophic, limnological, marginally hypertrophic, physico-chemical